Embodiments of the invention relate generally to a three-terminal switching device for controlling current in a conduction path, and more particularly to a plurality of switching devices in series connection within the conduction path.
Common three-terminal switches (such as metal-oxide-semiconductor field-effect transistor (MOSFET), insulated gate bipolar transistor (IGBT), and micro electromechanical system (MEMS) switches for example) typically include a source, drain, and gate terminal. The source terminal typically includes a connection that is responsive to application of a control voltage at the gate terminal relative to the potential at the source terminal to close the conduction path between the source terminal and the drain terminal. Three-terminal switches typically include ratings of maximum recommended voltage that may be applied across an open source terminal and drain terminal. Application of a voltage greater than the rating of maximum recommended voltage may result in a breakdown of the dielectric strength of the switch, or an arc from one terminal of the open switch to the other terminal of the open switch, thereby allowing an unintended current to pass through the conduction path and possibly damage the switch.
In order to utilize three-terminal switches to control the conduction path across a potential greater than the rating of maximum recommended voltage rating of any one switch, a switching circuit may include a plurality of switches placed in series such that the drain of one switch is series connected to the source of another. Such series arrangements typically include a resistive grading network that includes resistors in parallel with each switch, the resistors having high resistance values selected to reduce current flow and distribute the total open-circuit potential across each switch of the plurality when all of the switches are open.
However, such a series configuration results in a floating source voltage for each switch that is not connected to a reference, such as a ground. Therefore the voltage at the gate of each switch (relative to its respective source) that is required to close the connection between the source and drain differs by the amount of voltage distribution provided by the grading network. This is typically solved by the use of multiple isolated control voltage supplies corresponding to each of the three-terminal switches and provide to the gate and source of each three-terminal switch the appropriate voltage to close the connection between the respective source and terminal. Use of such multiple isolated supplies and their associated controls increases an overall complexity and cost of the switching circuit. Accordingly, there is a need in the art for a switching arrangement that overcomes these drawbacks.